<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
   <button class="begin">开启定时器</button>
   <button class="stop">关闭定时器</button>
   <script>
       var begin = document.querySelector('.begin');
       var stop = document.querySelector('.stop');
       var timer = null;//全局变量，null是空对象
       begin.addEventListener('click',function(){
          timer =  setInterval(function(){
            console.log('开始了');
           },1000)
       })
       stop.addEventListener('click',function(){
           clearInterval(timer);
       })
   </script>
</body>
</html>